A key step in the formation of the nervous system is the determination of proliferating neural progenitor cells to undergo differentiation into neurons and glia. Despite major advances in identification and characterization of neural progenitor cells (Placzek and Furley, Patterning cascades in the neural tube. Neural development. Curr. Biol., 6:526-29, 1996; Gage, F. H., Mammalian neural stem cells. Science, 287:1433-38, 2000; Kintner, C., Neurogenesis in embryos and in adult neural stem cells. J. Neurosci., 22:639-43, 2002; Schuurmans and Guillemot, Molecular mechanisms underlying cell fate specification in the developing telencephalon. Curr. Opin. Neurobiol., 12:26-34, 2002), the mechanisms that govern this determination are only partially understood.
The selective degeneration of specific types or classes of neurons of the central nervous system (CNS) underlies many neurological disorders. This realization has generated interest in defining populations of progenitor cells that, through manipulation of the differentiation process, may serve as replenishable sources of neurons and glia, and, therefore, may present an option for treating neurodegenerative and demyelinating disorders. Additionally, it is well recognized that neural tumors and other cancers develop when cells divide and grow uncontrollably. Thus, a means of manipulating the proliferation, differentiation and/or survival of tumor cells may provide a therapy for the treatment of cancers.
Neural degeneration may result from neurodegenerative diseases, CNS traumas, stroke, and the acquired secondary effects of non-neural dysfunction. Alzheimer's disease is a neurodegenerative disease characterized by a progressive, inexorable loss of cognitive function. The pathogenesis of Alzheimer's disease is associated with an excessive number of neuritic, or senile, plaques (composed of neurites, astrocytes, and glial cells around an amyloid core) in the cerebral cortex, and neurofibrillary tangles (composed of paired helical filaments). Approximately 4 million Americans suffer from Alzheimer's disease, at an annual cost of about $90 billion. The disease is about twice as common in women as in men, and accounts for more than 65% of the dementias in the elderly. While senile plaques and neurofibrillary tangles occur with normal aging, they are much more prevalent in persons with Alzheimer's disease. To date, a cure for Alzheimer's disease is not available, and cognitive decline is inevitable.
Demyelination is also a feature of many neurologic disorders. Demyelinating conditions are manifested in loss of myelin—the multiple dense layers of lipids and protein which cover many nerve fibers. Multiple sclerosis (MS) is the most prevalent demyelinating condition. In Europe and North America, an average of 40-100 people out of every 100,000 have MS. The disease affects approximately 250,000 people in the United States alone. Histopathologically, MS is characterized by inflammation, plaques of demyelination infiltrating cells in the CNS tissue, loss of oligodendroglia, and focal axonal injury. Typically, the symptoms of MS include lack of co-ordination, paresthesias, speech and visual disturbances, and weakness. Current treatments for the various demyelinating conditions are often expensive, symptomatic, and only partially effective, and may cause undesirable secondary effects. Corticosteroids represent the main form of therapy for MS. While these may shorten the symptomatic period during attacks, they may not affect eventual long-term disability. Long-term corticosteroid treatment is rarely justified, and can cause numerous medical complications, including osteoporosis, ulcers, and diabetes.
Approximately one million people are diagnosed with cancer each year, and many millions of Americans of all ages are currently living with some form of cancer. At some time during the course of their lifetime, one out of every two American men and one out of every three American women will be diagnosed with some form of cancer. Of the one million Americans diagnosed with cancer annually, 17,000 are diagnosed with brain tumors. Brain tumors invade and destroy normal tissue, producing such effects as impaired sensorimotor and cognitive function, increased intracranial pressure, cerebral edema, and compression of brain tissue, cranial nerves, and cerebral vessels. Drowsiness, lethargy, obtuseness, personality changes, disordered conduct, and impaired mental faculties are the initial symptoms in 25% of patients with malignant brain tumors. Treatment of brain tumors is often multimodal, and depends on pathology and location of the tumors. For malignant gliomas, multimodal therapy, including chemotherapy, radiation therapy, and surgery, is used to try to reduce tumor mass. Regardless of approach, however, prognosis for patients suffering from these tumors is guarded: the median term of survival after chemotherapy, radiation therapy, and surgery is only about 1 year, and only 25% of these patients survive for 2 years.
In particular, malignant astrocytic tumors occur in the human population at a frequency of 7 per 100,000 per year (Maher, et al. Malignant glioma: genetics and biology of a grave matter. Genes Dev., 15: 1311-1333, 2001; Rasheed, et al. Molecular pathogenesis of malignant gliomas. Curr Opin Oncol., 11: 162-167, 1999), making them the most common form of primary brain tumor. There is currently no effective curative therapy for patients with WHO classification Grade IV glioblastomas (also designated glioblastoma multiforme or GBM) and the average survival time from diagnosis is approximately 9-11 months (McLendon, et al. Tumors of central neuroepithelial origin., p. 307-571, 1998; Kleihues, et al. Histology Typing of Tumours of the Central Nervous System. Berlin: Springer-Verlag., 1993.).
Findings that neural progenitor/stem cells may be experimentally transformed into glioblastomas has supported the possibility that such tumors may arise from self-renewing progenitors that have lost the capacity for appropriate regulation of proliferation and survival (Dai, C. et al. Glioma models. Biochem. Biophys. Acta., 1551: M19-27, 2001). Indeed, GBMs are often associated with disregulation of pathways that control growth and survival including those involving p53, Rb, PTEN and growth factor receptors (reviewed by Collins (Collins, V. P. Brain tumours: classification and genes. J. Neurol. Neurosurg. Psychiatry, 75 Suppl 2: ii2-11, 2004). Additional novel regulatory genes may also contribute to blocking GBM cells from undergoing full differentiation and maintaining them in a state of uncontrolled growth.
In view of the foregoing, it is clear that many neural disorders are related to loss of cells, loss of myelin, or loss of cell control. An ability to regulate the differentiation of neuroprogenitor cells into various differentiated neural cells would provide supplies of neural cells that could be effective in treating such neural disorders. Additionally, the ability to regulate the growth and/or survival of tumor cells would be effective in treating an array of neoplastic disorders. However, prior to the present invention, manipulating whether or not neural progenitor cells differentiate into neurons and/or glia continue to divide and to remain as progenitor cells, as well as the general regulation of the growth and/or survival of stem cells and tumor cells has proved difficult.